1. Field of Invention
The present invention relates to a satellite signal reception device that acquires time information by receiving a signal from a GPS satellite or other positioning information satellite, to a timekeeping device that has this satellite signal reception device, and to a satellite signal reception method.
2. Description of Related Art
The Global Positioning System (GPS) for determining the position of a GPS receiver uses GPS satellites that circle the Earth on a known orbit, and each GPS satellite has an atomic clock on board. Each GPS satellite therefore keeps the time (referred to below as the GPS time) with extremely high precision.
In order for the reception unit of the satellite signal reception device that receives signals from the GPS satellites to get the time information from a GPS satellite, the receiver must receive and decode the TOW signal (the Time of Week or GPS time, information that is reset weekly and indicates the number of seconds from the beginning of the week). See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-H10-10251 (including abstract).
In order for the GPS receiver to receive this time information, it must first capture a signal from a GPS satellite orbiting the Earth. The GPS receiver must then receive and correlate the captured signals, and then perform certain operations to extract the time data.
More specifically, the GPS signal (signal from a GPS satellite) must be received through an antenna, converted to an intermediate frequency in the RF band, and then correlated by a baseband unit to extract the GPS signal. An operator then processes the extracted GPS signal to extract the time information. The GPS receiver generally has a plurality of reception channels, supplies a reference clock only to the reception channel used to capture and track signals from the GPS satellite needed to determine the current position, and thus reduces power consumption by the other reception channels. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-H07-311254.
In order to actually acquire the time information after receiving signals from the GPS satellite, the antenna unit, RF unit, baseband unit, and operating unit must be driven simultaneously.
In order to acquire the navigation message carried by the GPS satellite signal, the C/A (coarse/access) code must be interpreted.
The C/A code is a digital code containing irregularly inserted 0s and 1s known as a pseudo-random noise code (PN code). A different code pattern is assigned to each of the plural GPS satellites so that each satellite can be uniquely identified for signal reception and processing.
The satellite signal reception device usually receives the GPS signal through an antenna and converts the received signal to an intermediate frequency in the RF band. When the baseband unit then correlates the received signal, a plurality of correlators must be driven simultaneously.
As described above, the satellite signal reception device must simultaneously operate an antenna unit, RF unit, a plurality of correlators in the baseband unit, and a processor. The peak power requirement is therefore high. A large battery must be used to meet this peak power requirement. However, a clock, wristwatch, or similar timekeeping device incorporating such a satellite signal reception device is typically small. A large battery therefore cannot be used, and the timepiece or other electronic device thus using the satellite signal reception device therefore soon runs out of power.